As mentioned, use year's main role is determining banking deadlines. You can bank 100% of your points during the first 6 months of your UY. So, if you are able to determine a time of the year when you will usually be traveling, it is best to have your use year just before that time period. That way, if you ever do need to cancel, the points will return to the use year and then you will have most of the year to use or bank them.
Consider the alternative: You have a Sept Use Year and have booked an August Vacation. On June 30th, you have a change of plans and cancel your August vacation. The points will return to the use year, which only has two months left (July and August). You will not be able to bank the points and must use them or they will expire on Aug 31st.
Now, cancellations are likely rare unless you live locally. Also, it is difficult to know for certain what your main travel times are going to be for the next few decades. So, for most people, the Use Year is of minimal importance. That being said, I have an Oct use year and my main WDW (and DLR) travel will be Oct, Dec, and Jan for the most part (95%). I'm happy with my use year choice.